A conventional bicycle includes a frame having a normally non-driven front wheel and a rear driven wheel rotatably mounted thereon. The rear wheel conventionally has a driven element thereon which may comprise a rear chain driven sprocket cluster having several different size gears and a rear derailleur mechanism. A pedal driven crankset, that includes a pedal sprocket or sprockets, is mounted on the frame and serves as a prime mover for the drive wheel. A drive chain is trained around one of the pedal sprockets and one of the sprockets of the rear sprocket cluster. The rear derailleur mechanism is manually operable to shift the drive chain to train it about any desired one of the sprockets of the rear sprocket cluster. If the crank set includes two or three pedal sprockets, a front derailleur mechanism is also mounted to shift the chain to train it about any desired one of the pedal gears. Such shifting of the drive chain allows the operator to select an optimum sprocket ratio for driving the bicycle over the terrain on which it is being ridden. These sprocket ratios are commonly referred to as "speeds". A rear wheel drive bicycle may have rear wheel and pedal sprocket clusters that provide a large number of speeds with 10, 18 and 21 speed bicycles being in widespread use. The frame also includes a seat for supporting the bicycle rider and a front steerably mounted fork on which the front wheel is rotatably mounted. Handle bars are connected to the front steerable fork for steering the bicycle in known manner. The rear and front derailleurs each have controls mounted on the frame in a position to be conveniently reached by the operator to shift speeds while pedaling the bicycle.
Bicycles are increasingly being ridden off of paved roads and on rough terrain, which is steep, unpaved, frequently wet or muddy and covered by vegetation in places. Off road trails are particularly prone to have treacherously slippery sections caused by mud or uncut vegetation such as grass and weeds. Because of the rough and slippery terrain bikes are driven over, increased traction is desired. To achieve such traction, it is known to provide a multi-speed bicycle with a drive mechanism that allows for simultaneous and constant driving of both the front and rear wheels. This type of drive mechanism will be referred to as a constant dual wheel drive.
U.S. Pat. No. 559,934, issued May 12, 1896, to C. L. Cunningham; U.S. Pat. No. 4,029,332, issued Jun. 14, 1977, to Harry C. Davis; U.S. Pat. No. 4,479,660, issued Oct. 30, 1984, to Daniel J. Pattirson U.S. Pat. No. 4,895,385, issued Jan. 23, 1990, to Billie J. Becoat and U.S. Pat. No. 5,004,258, also issued to Billie J. Becoat, on Apr. 2, 1991, show various types of constant dual wheel drive mechanisms for always driving both of the wheels of a bicycle. These prior art constants dual wheel drive mechanism constantly drive both wheels and the bicycle rider is not able to selectively shift the bicycle between single or dual wheel drive operation. It is not advantageous to always constantly drive both wheels, and the inability to disengage the drive to one of the bicycle wheels reduces the efficiency of the bicycle drive under some operating conditions. For example, on flat terrain, it would be desirable to disengage the front wheel drive to eliminate frictional losses in the drive train which leads to the front wheel. However, with prior art designs this can not be done.
U.S. Pat. No. 5,052,705, issued on Oct. 1, 1991, to Thomas J. Ringle, discloses a selective single or dual wheel drive bicycle, which provides caliper actuated rear and front wheel idlers that are interconnected by a rotatable, flexible cable. By engaging both of the front and rear wheel calipers, drive power can be selectively transferred by the rider from the rear to the front wheel to provide dual wheel drive for the bicycle. This dual wheel drive relies on frictional engagement of each idler with its associated wheel rim. A friction drive is very inefficient and unreliable in wet conditions. Water, mud and tall succulent vegetation, which passes between the rim and the idler, all function as lubricants which will cause the idlers to slip relative to the rim. Such slippage makes it virtually impossible for an idler roller to efficiently receive drive power from the rear wheel rim by friction contact, and transfer such power to the front rim through the front frictionally engaged idler. Thus, friction drive mechanisms for transmitting drive power to the normally non-driven wheel becomes the least effective at the very time when dual wheel drive power is most needed; that is under wet slippery hilly conditions that tend to make the bicycle unstable and dangerous to ride.
Therefore, even though there has been almost 100 years of bicycle drive mechanism development following the issuance of U.S. Pat. No. 559,934, previously mentioned, a need still exists for a bicycle drive mechanism that will provide a selectively engageable positive transmission of drive power to either one or both wheels of the bicycle to avoid the long and well recognized short comings of prior art drive mechanisms.